(1) Field of the Invention
The present invention relates to a boiler system including a boiler and combustion amount control means for controlling combustion amount by the boiler.
(2) Description of the Related Art
In a conventional technique relating to control of a boiler in which in the case where steam or hot water is produced by combustion by a plurality of boilers, the number of boilers in charge of the combustion and combustion amount are calculated such that a steam pressure reaches a target value, so as to increase or decrease the combustion amount by a target boiler.
Alternatively, the boiler has widely used a feedwater preheater (i.e., an economizer) for previously heating (i.e., preheating) water to be fed (i.e., replenished) to the boiler. The feedwater preheater is adapted to, after there is provided a heat exchanger on a discharge passage for combustion gas from the boiler, for thermally exchanging heat of the combustion gas, previously heat (i.e., preheat) feedwater to the boiler with residual heat of the combustion gas in order to enhance the thermal efficiency of the boiler (i.e., boiler efficiency).
In another conventional feedwater preheater, the heat exchanger is disposed in a descendant passage extending downward from above on the discharge passage (i.e., in which the combustion gas descends downward from above). It is construed that one of reasons for the arrangement of the heat exchanger on the descendant passage resides in that condensed water (i.e., drained water) flows in the same direction as that of the descending combustion gas, and thus, the recovery effect of latent heat can be improved by a condensation effect.
In the boiler system including the above-described feedwater preheater for exchanging the heat with the combustion gas in the heat exchanger disposed on the descendant passage of the discharge passage so as to previously heat the feedwater to the boiler by the residual heat of the combustion gas, a low heat radiation loss by the boiler and a high boiler efficiency have been desired. The same goes for the case where the descendant passage is replaced with an ascendant passage in which combustion gas ascends upward from under as a passage on which the combustion gas flows in a vertical direction.